1. Field of the Invention
The present invention relates to a zoom lens, in particular, to a zoom lens applied to an image-capturing device such as DV (digital video), DSC (digital still camera) etc.
2. Description of Related Art
Referring to US Patent Publication No. US 2005/0099700A1, it discloses a compact zoom lens. The refractive powers of the four lens groups are positive, negative, positive and positive, respectively. When the zoom lens is zoomed from wide-configuration to tele-configuration, the positions of the first lens group and the third lens group are fixed and the second lens group is moved to image side. The size of the zoom lens is compact, but the zoom ratio (3×) is low.
Referring to U.S. Pat. No. 5,909,318, it discloses a lens image system that includes a first lens group, a second lens group and a third lens group. The first and the second lens groups are movable, and the third lens group can be moved a little (almost fixed). The first lens group is composed of four pieces of lens', the second lens group is also composed of four pieces of lens, and the third lens group includes a color filer and an image lens. However, the number of the lens is too many (11 pieces of lens), so that the size is large and the cost is high.
Referring to U.S. Pat. No. 5,285,316, it discloses four lens groups, and the refractive powers of the four lens groups are positive, negative, positive and positive, respectively. The four lens groups are composed of ten pieces of lens, the third lens group is fixed, two aspherical surfaces are provided. However, the second and/or fourth lens groups are movable, so that the variation of the focal length is insufficient. Hence, when pixel and brightness are increased, the whole size of the zoom lens is increased and the image multiple and resolution are limited. In addition, the zoom lens needs many numbers of lenses, so that the cost is increased.
Referring to U.S. Pat. No. 6,714,355, it provides 3× zoom ratio and 2.8 Fno, and it discloses four lens groups, and the refractive powers of the four lens groups are positive, negative, positive and positive, respectively. The four lens groups are composed of ten pieces of lens, the third lens group is movable, and one aspherical surface is provided. However, the zoom ratio is insufficient, the aperture value is too high and the number of lens is too many, so that the zoom lens cannot be compacted.
Referring to U.S. Pat. No. 7,379,249, it provides five lens groups composed of ten pieces of lens that are made of glass, so that the cost is increased. In addition, it provides 2.78˜4.02 Fno, so that the variation of aperture value is large. The aperture ratio of wide-configuration to tele-configuration is (4.02/2.78)2=2.09, so that the aperture value at tele-configuration (*4.02) is smaller than the aperture value at wide-configuration (*2.78). In other words, bigger FNO variation range 2.78˜4.02 in design, so the input light different is bigger also, the ratio is 2.09 times.
Referring to U.S. Pat. No. 7,453,647, it provides four lens groups composed of nine pieces of lens that are made of glass, three of the lens groups are movable and the total length of the zoom lens is 78.5 mm, so that the cost is increased. In addition, it provides 2.53˜4.74 Fno, so that the variation of aperture value is large. The aperture ratio of wide-configuration to tele-configuration is (4.74/2.53)2=3.48, so that the aperture value at tele-configuration (*4.74) is smaller than the aperture value at wide-configuration (*2.53). In other words, bigger FNO variation range 2.53˜4.74 in design, so the input light different is bigger also, the ratio is 3.48 times.